Fuel cell assemblies employing a plurality of individual fuel cell modules are well known. Each module has an anode and a cathode. In a proton-exchange fuel cell, the anode and cathode are separated by a catalytic proton exchange membrane (PEM). The modules in the stack typically are connected in series electrically through bipolar plates to provide a desired total output voltage. Fuel in the form of hydrogen and water vapor, or hydrogen-containing mixtures such as “reformed” hydrocarbons, is flowed through a first set of reaction channels formed in a first surface of the bipolar plate adjacent the anode. Oxygen, typically in the form of air, is flowed through a second set of reaction channels formed in a second surface of the bipolar plate adjacent the cathode.
In a PEM fuel cell, hydrogen is catalytically oxidized at the anode-membrane interface. The resulting proton, H+, migrates through the membrane to the cathode-membrane interface where it combines with ionic oxygen to form water. Electrons flow from the anode through a load to the cathode, doing electrical work in the load.
In fuel cells, especially PEM fuel cells, a long-term electrical continuity problem is well known in the art. Metals typically used to form bipolar plates, for example, aluminum or stainless steel, either corrode or form high-resistance oxide passivation layers on the surface of the bipolar plates because of electrochemical activity at these surfaces. These oxide layers limit the current-collecting ability of the bipolar plates, significantly lowering the efficiency and output of a fuel cell. Further, due to the intrinsic properties of a preferred membrane, the environment for a bipolar plate is relatively acidic; pH values of 3.5 or lower are common. Further, cyclic voltage exists between the anode and cathode as the load on the fuel cell varies. The combination of low pH and cyclic voltage requires that the bipolar plate be very corrosion resistant, especially on surfaces making contact with the anode and cathode.
In the prior art, it is known to form bipolar plates of graphite/polymer composites, which are highly resistant to corrosion. Such materials are relatively brittle but have relatively high bulk resistivity. Therefore, such bipolar plates must be relatively thick for structural integrity and, contrastingly, relatively thin because of its high bulk resistivity. Further, such bipolar plates may be difficult and expensive to form and may be easily damaged during assembly of a fuel cell assembly.
In the prior art, bipolar plates formed of metal are known. Metals are advantageous over graphite/polymer as having a relatively low bulk resistivity, yet in being strong enough to form very thin plates, thereby reducing substantially the size and volume of a fuel cell assembly, and are relatively easy to form into channeled plates. However, as noted hereinabove, most metals or alloys are not able to provide sustained corrosion-free contact with the anode and/or cathode of a PEM fuel cell. It is known in the art to coat the entire surface of a metallic bipolar plate with noble metals such as gold or platinum to prevent corrosion, but such coatings require so large an amount of noble metals that this approach is cost-prohibitive.
What is needed is a simple and cost-effective means for maintaining electrical conductivity of the electrical-contact surfaces of a bipolar plate.
It is a principal object of the present invention to provide an improved bipolar plate which is simple and inexpensive to manufacture and which reliably maintains electrical conductivity and continuity of the surface during use in a fuel cell.
It is a further object of the invention to increase the durability and reliability of a fuel cell.